1. Field of the Invention
This invention relates generally to protective relay apparatus and methods for protecting ac electrical power transmission lines, and more specifically, to high speed protective relay apparatus and methods utilizing traveling waves to detect a fault on the transmission line.
2. Description of the Prior Art
Three-phase electrical transmission lines and power generating equipment must be protected against insulation faults and consequent short circuits or drops in shunt resistance which could cause collapse of the power system, serious and expensive apparatus damage, and personal injury. For instance, such a fault condition is caused by lightning-induced flashover from a transmission line to ground or between adjacent transmission line conductors. Under such a faulted condition, line currents can increase to several times the normal value thereby causing loss of synchronism among generators and damaging or destroying both the transmission line and the attached equipment. To avoid equipment damage and collapse of the entire power system, faulted apparatus on the main transmission line must be isolated from the network in 0.1 to 0.5 seconds. The isolation time limit must allow for the operation of large circuit breakers interrupting up to 80,000 amperes and the completion of backup operations if these primary protective devices fail to function properly. To allow sufficient time for circuit interruption, location of the fault must be determined in approximately 8 ms to 20 ms. It is the function of the protective relays, which continuously monitor ac voltages and currents, to locate line faults and initiate isolation via tripping of the appropriate circuit breakers.
The direction to a fault with reference to a measuring location on a transmission line is usually determined with the aid of distance relays. These relays usually incorporate electromechanical or electronic elements which require substantially sinusoidal power-frequency input signals to function correctly. When a fault occurs on the transmission line, the power-frequency signals are distorted by the transient traveling waves generated by the fault. Since the distorted power-frequency signals are not suitable for detection by a distance relay, operation of the protective relay must await decay of the distortion effects, i.e., the traveling waves. This decay is a comparatively slow process. Alternatively, frequency filters may be used to filter the effects of the transient traveling waves thereby enabling the power-frequency components to be evaluated by the protective relay at an earlier time. However, the filtering action also limits the response time of the protective relay.
Since increasing the speed of fault detection and the consequent fault clearing improves system stability, it has been proposed that the initial changes in transmission line voltage and current caused by the traveling waves generated by a fault be utilized to detect and clear faults. The resulting clearing time is faster than possible using the power-frequency signals. Additionally, to clear a fault with minimum disturbance to the generation and transmission system requires the protective relay to determine both the direction to the fault and the specific phase conductor on which the fault is located.
Two protective relays, located at opposite bus terminals of a transmission line, with the intervening transmission line define a protected line segment. When the charged transmission line is faulted to ground at a point between the protective relays an incident traveling wave of voltage and current is launched from the fault in both directions toward the protective relays. For clarity in the subsequent discussion, it is to be assumed that a step voltage change, as caused by a fault to ground, produces a positive current into the protected line segment. It is well known in the art that the voltage and current of such an incident traveling wave have opposite signs and are related by the equation .DELTA.I=-Y.sub.0 .times..DELTA.E, where .DELTA.I is the change in current from the steady-state current, .DELTA.E is the change in voltage from the steady-state voltage, and Y.sub.0 is the characteristic admittance of the transmission line. If the current and voltage at both protective relays have opposite signs, this indicates the fault is on the protected line segment and both protective relays trip the appropriate circuit breakers.
For external faults, i.e., outside the protected line segment, the current and voltage at the protective relay nearest to the fault have the same sign, while at the furthest protective relay they have opposite signs. At the nearest protective relay the current and voltage are related by the equation .DELTA.I=Y.sub.0 .times..DELTA.E, where .DELTA.I is the change in current from the steady-state current, .DELTA.E is the change in voltage from the steady-state voltage, and Y.sub.0 is the characteristic admittance of the transmission line. Under these conditions the protective relay nearest the fault transmits a block signal to the other protective relay. The block signal prevents the latter from tripping although the current and voltage at this protective relay are opposite in sign.
One prior art technique for implementing the above fault detection scheme is disclosed in U.S. Pat. Nos. 3,878,460; 3,956,671 and 4,296,452. After filtering the current and voltage transient waves to remove the steady-state ac power frequency signal, level detectors and sign indicators are used to determine the sign relationship of the current and voltage traveling waves and produce the tripping and blocking signals if the current and voltage traveling waves exceed a predetermined threshold.
U.S. Pat. No. 4,287,547 (and a related article "A Fundamental Concept for High-Speed Relaying;" by M. Vitins; IEEE Transactions on Power Apparatus and Systems; Vol. PAS-100, No. 1; January 1981; pp. 163-168) discloses a more sophisticated apparatus for detecting a fault from current and voltage traveling waves. Using the current and voltage surge signals, representing the change in current and voltage from the steady-state condition, as X and Y coordinates respectively, a trajectory is plotted in the X-Y plane. The trajectory represents the change in the current and voltage transient signals over time. Threshold boundaries are also established in the X-Y plane, and depending on the boundary crossed, a fault detection determination can be made and appropriate tripping and blocking signals produced. The threshold boundaries are either predetermined or functionally related to the current and voltage surge signals for more accurate fault detection.
U.S. Pat. No. 4,371,907 assigned to the assignee of the instant invention also discloses a trajectory-plotting means to determine the location of a fault. However, in this patent the current deviation signal is differentiated before plotting on the X-Y plane. As a result, for typical bus terminations the eliptical trajectory that is formed in the previously discussed patents is transformed to a straight line thereby providing more accurate detection of the fault's location.
Other prior art U.S. patents exemplifying fault detection using traveling waves include, U.S. Pat. Nos.: 3,590,368; 4,063,160; 4,063,162; 4,063,163; 4,063,164; 4,063,165; 4,063,166; and 4,183,072. U.S. Pat. Nos. 4,063,160; 4,063,162; 4,063,166; 4,063,165 and 4,963,166 produce two signals representative of the forward and incident waves at a test location. From a determination of the time displacement between these oppositely moving traveling waves the direction and distance to a fault can be determined. U.S. Pat. No. 4,063,164 teaches monitoring the respective phase positions of two oppositely moving traveling waves to detect short circuits on the transmission line.
U.S. Pat. No. 4,183,072 teaches yet another protective relay and apparatus for detecting a fault on a transmission line. Essentially, for a single wire transmission line the apparatus compares the current and voltage of the traveling wave as it passes through one of the line terminals with a current and voltage of the same traveling wave as it passes through the other line terminal, after having propagated over the intervening transmission line. The current and voltage will always be equal unless a fault has occurred on the transmission line between the two line terminals. For a three phase transmission line the three-phase currents and voltages are transformed into their modal components for comparison of the modal voltages and currents. Again, the modal voltages and currents will be equal unless a fault has occurred on the line. Similarly, the apparatus of U.S. Pat. No. 3,590,368 compares voltages and currents at two locations after providing for the attenuation and delay associated with propagation over the connecting transmission line. On a three phase basis this is accomplished by transforming the phase voltage and currents into their modal components and comparing the resulting modal voltages and currents. Three comparison equations are developed to allow for identification of the faulted conductor.
Several articles discussing ultra-high speed relaying using traveling waves are also available in the literature. An article entitled "Ultra-High Speed Relay for EHV/UHV Transmission Lines-Development, Design and Application", by M. Chamia and S. Iberman appearing in the IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, No. 6, November/December 1978, page 2104-2116 discloses a protective relay for comparing the polarity of the current and voltage at each line end. For an internal fault both line ends will have changes in current and voltage which are opposite in sign; for an external fault one line end will have changes of equal sign. Further discussion of this protective relay appears in the article, "Ultra-High Speed Relay for EHV/UHV Transmission Lines-Installation-Staged Fault Tests and Operational Experience" by M. T. Yee and J. Estzergalyos, in the IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, No. 5, September/October 1978, pages 1814-1825.
Two additional related articles discuss fault detection using traveling waves: "Fault Protection Based on Travelling Wave Theory-Part I Theory," presented at the IEEE Power Engineering Society Summer Meeting, July 17-22, 1977; and "Fault Protection Based on Travelling Wave Theory-Part II Sensitivity Analysis and Laboratory Test," presented at the IEEE Power Engineering Society Winter Meeting, Jan. 29-Feb. 3, 1978; both articles are authored by Toshio Takagi, Jun-ichi Baba, Katsuhiko Uemura, and Toshiaki Sakaguchi. The articles teach fault detection by comparing the traveling waves at local and remote ends of the transmission line. Using a modal transformation, the same ideas are shown to be applicable to a multiple-conductor transmission line.
Compared with the prior art, the present invention utilizes a new frequency-dependent characteristic admittance model of the transmission line. In addition, it makes novel use of the modal current and voltage propagation vectors at a single line terminal to detect a fault and to determine on which phase conductor the fault is located. These and other advantages of the present invention are discussed below in the description of the preferred embodiment.